Process of hot coating metal strip



Nov. 11, 1947.

E. MARSHALL PROCESS 01'' HOT COATING IIETAL STRIP Original Filed. April' 15, 1942 u. n mm m. I A w Patented Nov. 11, 1947 rnocnss or'no'r coarmc- METAL s'rarr William E. Marshall, Middletown, Ohio, assignor to The American Rolling Mill Company, Middletown, Ohio, a corporation of Ohio Original application April 15, 1942, Serial No. 438,999. Divided and this application October 16, 1945, Serial No. 622,620

1 Claim. 1

This is a division of my copending application entitled hot coating of metal strip and the like,

Serial No. 438,999, filed April 15, 1942.

My invention relates to improvements in the coating of metals, usually iron or mild steel, with a layer of molten metal. It has special application to treatments for the base metal where it is in the form of a continuous strand, strip, wire or the like. It will be evident, however, from my description that many of the principles of the invention may be applied to separate sheets if suitable means are provided to carry the sheets through the treatment apparatus. the treatment of strand type materials at very high speeds has given rise to new problems in the metal coating industry, and the exemplary embodiments of my invention will be described in connection with the treatment of such materials.

My invention relates to the coating of metal bases with pure aluminum, alloys of aluminum and zinc, alloys of aluminum and silicon with or without magnesium, pure zinc, the various alloys way of example-upon lead for the purpose of isolating the coating bath itself froman entrance flux. The endeavor is not always successful, for the reason that some materials, such as aluminum and silicon, are capable of dissolving in molten lead to some extent. Where these materials form undesirable compounds with constituents of the flux, some diillculty may be encountered in this direction. Solutions for such difficulties are taken up in part in my'copending application entitled The Cleaning of metal strip at high speeds, Serial No. 439,846, filed April 21, 1942, together with teachings relating primarily to the proper fiuxing of'strand type materials at high speeds. But there are other problems connected with the use of a molten supporting bath such as a bath of lead, in combination with a coating bath and a fluxing bath. It is frequently desirable that the fluxing bath be maintained at a temperature different from the temperature-of the coating bath. A flux of zinc chloride and ammonium chloride, frequently used in galvanizing metal, is, if properly handled, an'

eflicient cleaner; but if it is maintained at too high a temperature, a great deal of the, ammonium chloride will be lost through vaporization,

Nevertheless,

(Cl. 117-52) 2 p and the flux will have to be replenished frequently. It is possible, by adding such a salt as sodium chloride to the exemplary flux mentioned, to cut down the vaporization of ammonium chloride at higher temperatures; but where coating, for example, with pure aluminum, the temperature of the coating bath ought to be around 1200" F. whereas such a flux will not stand that'temperature. Hence, one of the objects of my invention is to provide a structure and a mode of operation which will permit fluxing at one temperature and coating at another temperature.

Another object of my invention is to provide a structure and mode of operation in which the strand-form material may be treated at differeflects, such for example as the effect of annealing the material at temperatures which aretoo high for the flux and may even be too high for the coating bath. I

It is an object of my invention to provide an apparatus and a mode Of procedure minimizing diffusion of ingredients of the coating bath through the-supporting bath, and at the same time minimizing the tendency of the material being coated to carry over into the coating bath particles of flux or dross.

It is still another object of my invention to provide an apparatus and a'mode of procedure whereby the temperature of the molten coating bath is rendered independent of the fluxing temperature and other temperatures throughout the apparatus, for the attainment of special effects. By way of example, even. in coating iron or mild steel with zinc, it is frequently desired to avoid the production of a spangle, especially where the coated material is to be given a subsequent treatment to increase its receptivity to paint, enamel and the like. It is known that if the zinc bath can be heated to a temperature of 950 F. or over, and the other conditions are proper, a spangle free coated strip may be produced. Such a high exit temperature, however, is detrimental to the ordinary fluxes, for which reason the operation has heretofore been unsatisfactory.

It is a further object of my invention to provide an apparatus and a mode or procedure whereby the entrance flux can be maintained at a higher temperature than that of the metal coating bath. This makes it possible to employ fluxes having very high melting points, and to coat with metals having very low melting points.

' These and other objects of my invention which will be set forth hereinafter or will be apparent 55 to one skilled in the art on reading these specifications, I accomplish by that certain construction and arrangement of parts and in that procedure of which I shall now describe certain exemplary embodiments.

Reference is made to the drawings, which are diagrammatic in nature and which illustrate several embodiments of my invention.

Figure 1 is a diagrammatic representation of the invention in a simple form showing also means for precleaning the strip.

Figure 2 is a view of a coating apparatus of somewhat more elaborate form.

Essentlally in the practice of my invention, I employ an elongated vessel in which I maintain a bath of molten lead or lead alloy. The vessel has an entrance part, in connection with which I maintain a flux, and an exit part in connection with which I maintain the bath of molten metal which is to form the coating on the strip. The vessel itself is so greatly elongated that a temperature gradient may be maintained in the lead bath. I prefer also to minimize the diffusion of heat through the lead by providing suitable baflles to interrupt convection currents, and to minimize the conduction of heat, and where possible I prefer to provide that portion of the lead bath which extend between the entrance and the exit in the form'of a long and narrow neck.

Referring to the drawings, in Figure 1, I have shown a strip I being withdrawn from a coil 2 by means of pinch rolls 3. The strip may be passed through an oxidizing furnace 4, which will burn from thesurfaces of the scale-free material any oil or other combustible substances and which will form'on those surfaces preferably, a thin, controlled coating of oxide varying in color from straw to blue and into the gray.

Over a roll 5, the strip enters a pickling bath 8 contained in the tank I, where it is held down by roll 8. The oxidizing furnace and pickling bath referred to are exemplary of suitable cleaning means for the strip; but the strip may be otherwise cleaned as desired.

After leaving the pickling bath, the strip is shown as passing over a roll 9 and downwardly through a flux box containing a flux ll, into the lead bath l2 contained in a suitable vessel It. This vessel is of elongated form and I have 'shown a wide, deep, refractory partition H ex:

tending downwardly into the lead throughout the entire midsectionof the pot. This partition confines the lead between the entrance and exit portions to a narrow throat marked I5, through which throat the strip passes, being guided by rolls l6 and i1. Over the latter roll the strip passes upwardly through the metal coating bath l8 which is floating on the lead. The partition it serves to isolate the coating bath from the entrance portion of the lead bath. The strip may emerge through exit rolls l9 and be carried away over a roll 20.

The provision of the elongated throat l5, as demarked by the partition l4, enables me, by way of example, to'heat the exit portion of the bath to a very high temperature as compared with the entrance portion, as for example by heating means diagrammatically indicated at 32. By applying heat only to the exit portion, a very substantial temperature gradient may be attained because of the diminished transfer of heat from the exit portion to the entrance portion of the bath. Thus, while the temperature at the entrance portion may be maintained low enough to preserve the qualities of a flux having volatile constituents, the temperature of the molten coating bath may be raised far above such a temperature for any of the purposeswhich have been given above. Thus, it is entirely possible to maintain a bath of molten aluminum on a lead bath at a temperature of say, 1250, while the fluxing temperature at the entrance of the bath is Bay, 600 to 800. Moreover, aluminum is very slightly soluble in lead, and the long constricted throat l5 minimizes the diffusion of the aluminum toward the entrance end of the bath thereby greatly cutting down the difliculties which may arise due to the interaction of aluminum and the flux.

The same elongated throat l5 enables me to operate the entrance portion of the bath, which carries the flux, at a higher temperature than the exit portion which carries the coating metal. Thus I am enabled to carry out special eflects such as the use of very high melting fluxes or very low melting coating metals. When operating in this manner, it may not be necessary to apply heat to the coating end of the bath because of the heat carried into it by the strip. In some instances it may even be necessary to apply cooling means to the exit end of the coating bath to avoid heating above the desired coating temperature.

In Figure 2, I have shown a vessel which is still more elongated and may be elongated to any extent desired. This vessel is indicated at 2|. The strip 22 enters it through a flux box 23, and under rolls 24 and 25 it passes along the bottom of the vessel substantially throughout its entire length. Where the vessel is very long, a plurality of support rolls may be arranged along the bottom to prevent the strip from dragging. The vessel is provided throughout its length with a series of refractory partitions 26, 21, terminating short of the bottom of the vessel, and serving to divide the lead bath 28 into a series of compartments between which there will be a minimum of flow by convection and a minimum of heat transfer by conduction.

At the exit end of the vessel, there is a partition 29 servin to confine the coating bath 30, through which the strip passes as it emerges from the vessel. Again, I may employ exit rolls 3|.

The vessel 2| may be made, as long as desired and there maybe as'many as desired of the partitions26, 21. Instead of employing one long vessel, I may employ a plurality of vessels arranged in a line; and by using suitable guide rolls, I may conduct the strip from one vessel to another. In my copending application referred to above, I have'shown how the strip may be carried from one vessel to another, either through a molten bath of flux or through a hood in which is maintained a non-oxidizing atmosphere. It will be understood that these are variants as respects the invention herein claimed.

The point, however, of the structure of Figure 2 and its variants is, that the temperature of the lead bath in its various parts or of the several lead baths where more than one are employed, may be maintained at substantially any desired temperature differentials. Further, the'path of travel of strip through the lead bath is very greatly elongated so that where I desire to do so, the bath may be caused to anneal the strip or strand at substantially any desired annealing temperature for substantially any length of time consonant with the speed of travel of the strip and the space limits in the coating plant. By

way of example, using the structure of Figure 2, I

may maintain a temperature of say 600 to 850 F. at the entrance end of the bath, but I may cause this temperature intermediate the ends of the lead bath to rise to say, '1'250' F, or higher by heating means (a burner being diagrammatically indicated at 33) falling again toward the exit end of the bath say, to 600 to 850 F. where I am coating with low melting point metals. Where, however, the coating metal is one, like alluninum, whichrequires a high temperature, such a temperature may be maintained at the exit end of the bath, and the temperature there may, if desired, be greater than the temperature elsewhere.

Modifications may be made in myinvention without departing from the spirit or it. Having now fully described my invention in certain exemplary embodiments, what I claim asv new and desire to secure by Letters Patent is:

A process of coating ferrous base metal with molten coating metal which comprises leading the metal to be coated through a bath of flux into a bath of molten supporting metal and out of said molten supporting metal through a bath of molten coating metal, the bath of molten supporting metal being divided into entrance and exit portions separated to minimize heat transfer and connected together to provide a passageway for the metal to be coated, which passageway includes a constricted throat to minimize diffusion, said flux being supported on said entrance portion and said molten coating metal being supported on said exit portion, and applying heat to said bath of molten supporting metal to provide a temperature in said entrance portion substantially between 600 and 850 F. and to provide a temperature in said exit portion and in the molten coating metal supported thereby of at least substantially 1200 F., said molten coating metal being of a class consisting of aluminum and its alloys, and said molten supporting metal comprising lead.

,WILLIAM E. MARSHALL.

REFERENCES CITED Y The following references are of record in the file of this patent:

UNITED STATES PATENTS Steckel May 9, 1933 

